GB2024459A

GB2024459A - Control device for variable displacement fluid pumps or motors

Info

Publication number: GB2024459A
Application number: GB7919849A
Authority: GB
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 1978-06-13
Filing date: 1979-06-07
Publication date: 1980-01-09
Also published as: FR2428751A1; DK233979A; DE2825827A1; IT1118778B; FR2428751B1; IT7968264A0; DE2825827C2; US4275996A; JPS5540283A; GB2024459B

Description

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GB 2 024 459 A 1

SPECIFICATION

A control device for variable displacement fluid pumps or motors

The invention relates to a control device for adjustable fluid pumps or motors, comprising a servo-piston which adjusts a servo-member of the pump, is arranged in a cylinder space and is subjected to a resetting force, and a valve which has a slide connected to an actuating element and a slide bore connected to the servo-piston and connects the pressure chamber of the servo-piston to a chamber at pump or motor pressure in its one operating position, to a chamber at discharge presssure in its other operating position and blocks in its neutral position, a central position of the slide corresponding to a position of the servo-member in which there is zero pump feed.

Numerous pumps are known in which the quantity that is fed can be changed with the aid of a servomember. These include axial and radial piston pumps with adjustable stroke, gear and vane pumps with variable compression and pumps with a variable rotary driving speed. Of particular advantage are those pumps in which the fed volume can be changed and the pumping direction even reversed by altering the eccentricity between two relatively rotating pump portions while maintaining the rotary driving speed. The servo-members for such pumps can generally be manually adjusted. It is also already known to adjust such servo-members with the aid of a servo-piston and a sleeve valve so that comparatively small control forces will be sufficient. In these control devices, the slide merely has a purpose in conjunction with the stated sleeve valve.

In adjustable pumps it is difficult to achieve a position in which the fed amount is absolutely zero. There is generally a small amount of feeding in the one or other direction. Intensive internal heating occurs in many adjustable pumps if the pump runs in the neutral position for a prolonged period.

The invention is based on the problem of providing a control device of the aforementioned kind with the aid of which the feed can be reduced to zero and heating of an adjustable pump can be reduced.

This problem is solved according to the invention in that the slide forms a second valve which, in the central position, interconnects the chamber at pump pressure and the chamber at discharge pressure.

In this construction, the pump pressure conduit is connected to the tank by way of the control device in a dead zone at both sides of the zero feed position. Any liquid that might be fed by the pump is immediately diverted into the tank without any substantial build-up in pressure. The power loss of the pump is correspondingly low. Heating is also at a minimum. On the contrary, a small throughput is even desired because it cools the pump. All this requires no marked additional expense because the slide takes on the function of the second valve.

Further, the slide may be movable out of the central position against the force of a first resetting spring in the one direction and against the force of a second resetting spring in the other direction, each spring being prestressed. In the absence of an external control force, the slide therefore immediately returns to its central position so that the pump will be longer feed and will therefore always operate at a low loss if there is no corresponding control command.

In a preferred embodiment, it is ensured that the slide sealingly passes through a bore between the chamber at pump pressure and the chamber at discharge pressure and has an axial passage at the periphery that is longer than the bore. The second valve function therefore only requires a very slight modification of the control device.

In particular, the bore can be formed in a sleeve which is arranged with radial play and has its end face sealingly pressed by a resetting spring and the differential pressure applied thereto against a dividing wall between the two chambers. In this way one can prevent jamming of the slide that might arise if for reasons of production it is not possible accurately to align the slide bore and the bore in the second valve.

An accurately defined central position is achieved by an inner collar fixed with respect to the housing an outer collar on the slide fitting through the circular section of the inner collar, and-two annular discs at both sides of the collars that serve as supports for the resetting springs and cover both collars.

If the inner collar is provided on an axially screw-connectible housing portion, one can thereby change the central position and accurately adapt it to the zero feed position of the pump.

In a very simple construction, the collars are formed by circlips sprung into a groove fixed with respect to the housing and a groove on the slide.

The present invention also provides a control device comprising a servo-motor for controlling the displacement of a pump and a valve having a valve member and two chambers, one of which valve chambers is for connection, in use, to fluid at pump pressure and the other valve chamber is for connection to tank, the valve having two operating conditions in which, respectively, the servo-motor is influenced by the pressure in the said one or the said other valve chamber and a neutral condition in which that influence is removed, wherein the arrangement is such that when, in use, the servomotor has placed the pump in its zero displacement condition, the valve is in a condition in which the two valve chambers are connected.

A control device constructed in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:—

Fig. 1 is a diagrammatic illustration of the control device connnected in circuit with a variable displacement pump,

Fig. 2 is a longitudinal section through the control device,

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GB 2 024 459 A 2

Fig. 3 is a graph of resetting spring force against displacement of the valve slide and

Fig. 4 is a graph of pump delivery against displacement of the valve slide.

Referring to the accompanying drawings, a -jq variable displacement pump 1, (which may, for example, be a radial piston pump of the type described in British Patent Specification No.

1,289,500 to which attention is directed for details) comprises a servo-member 2, movement 75 of which varies the eccentricity between a rotating pump portion, e.g. a radial piston carrier or rotor,

and a fixed pump portion, e.g. a piston track ring.

In this way, the delivery of the pump can be varied from maximum in one direction, through zero, to a go maximum in the other direction. Both pump connections 3 and 4, which may be connected in conventional manner to a load (not shown) and a tank, communicate by way of a respective check valve 5, 6 with a control conduit 11 which is g5

always at pump pressure.

The servo-member 2 is connected to a piston

12 of a servo-motor which has a cylinder 13 and to a small auxiliary piston 25 which is accommodated in a cylinder 26. A resetting 90 pressure chamber 14 in this cylinder is always connected to the control conduit 11. On the other hand, the actual working chamber 15 of the servomotor communicates by way of a conduit 16 with a directional control valve 17. A housing sleeve 32 95 of the valve 17 is mechanically coupled to the servo-piston 12 and a slide or valve member 31 is subjected to two resetting springs 18 and 19 and can be adjusted by an actuating element 20.

In the illustrated neutral position of the valve, 100 the conduit 16 and thus the working chamber 15. are blocked from communication with either the conduit 11 or a conduit 21 (connected to tank 9). The position of the servo-element 2 is thus fixed.

In one operating position of the valve, the working 105 chamber 15 is connected to the pump pressure conduit 11 and in the other operating position to the discharge conduit 21. In this way the servo-member 2 can be adjusted in the one or other direction. 110

Fig. 2 shows the construction of the control device 22 in detail. In a housing 23, a pin 24 connected to the servo-member 2 is disposed between the actual servo-piston 12 and the auxiliary piston 25. The latter is subjected to a 115 spring 27. The housing 23 also forms the cylinder

13 and is closed at the bottom by a cover 28 and at the top by a cover 29.

A valve stem 30 provides at its outer end an actuating element 20 and at its opposite end a 120 valve spool 31 which slides in a bore provided in a sleeve 32. This sleeve is arranged with radial play in a bore 33 of the servo-piston 12 which forms part of the working chamber 15 and its lower end face is sealingly pressed against an annular 125

dividing wall 35 of the piston by a spring 34 and by the differential pressure acting on it. In the inner wall of the sleeve 32 there is an annular groove 36 which, on the one hand, communicates with the pressure chamber 15 by way of a throttle 130

bore 37 in the sleeve and on the other hand, upon relative displacement between the sleeve 32 and valve member 31, is connected to a chamber 40 at discharge pressure by way of transverse bores

38 in the valve member 31 and an axial passage

39 in that member, or to a chamber 44 at pump pressure by way of transverse bores 41 in the valve member 31, an axial passage 42 in the stem 30 and further transverse bores 43 in that stem.

The chamber 44 is separated from the pressure chamber 15 by a dividing wall 45 (formed by the cover 29) which has a bore 46 sealingly abutting the periphery of the stem 30. A third dividing wall 47, formed as a housing portion 55, separates the chamber 44 from a second chamber 48 at discharge pressure. Here, again, there is a sleeve 49 which sealingly surrounds the stem 30 but is arranged radially with play within the housing portion 55. This sleeve 49 has its end face sealingly pressed against the dividing wall 47 by the resetting spring 18 and the differential pressure applied thereto. At the other end, the chamber 48 is sealed by an 0-ring 50. Axial valve passages or recesses 51 are provided in the stem 30; they are longer than the sleeve 49 is high so that in the 'central' position (not shown) of the stem 30, that is to say its position when the pump is in the zero-displacement condition, a short-circuit occurs between the chambers 44 and 48 by way of these axial passages 51.

The chamber 48 is connected by way of housing_ passages 52 to the chamber 40 and the latter is connected to the tank conduit 21. The chamber 44 and the chamber 14 are likewise connected to the pump pressure conduit 11 by way of housing passages 54.

it will be evident from the description so far that movement of the stem 30 provides a dual valve action byvirtue of its cooperation with the sleeve 32 and the sleeve 49.

The housing portion 55 is screw-threadedly inserted in the cover 29. In an inner annular groove, it carries an inner collar 56 in the form of a circlip. In an outer annular groove, the stem 30 carries an outer collar 57 in the form of a circlip of which the external diameter is smaller than the internal diameter of the collar 56. The resetting springs 18 and 19 are associated respectively with an annular disc 58 and 59 which engage over the collars 56 and 57. In the central position of the stem 30 the collar 57 is in the plane of the collar 56. As a result movement, upwards or downwards, of the stem from the central position is against the force of spring 18 or the spring 19. By turning the housing portion 55, the central position can be accurately adapted to the zero displacement condition of the pump.

Fig. 3 illustrates how the force (F) of a respective one of the two resetting springs 18 and 19 is associated with each direction of displacement, S, (upwards or downwards) of the stem 30 and that these springs are prestressed in the central position (S = 0) so that the stem is returned to its central position in the absence of external control forces.

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GB 2 024 459 A 3

Fig. 4 shows that the delivery, Q, of the pump has a dead zone in the region of the central position. This arises out of the short circuit between the pump pressure side and the tank by 50 reason of the axial passages 51. In the region of this dead zone, the liquid is circulated externally of the pump so that continual cooling takes place and the pump does not become too hot as a result of internal circulation. 55

Claims

1. A control device comprising a servo-motor for controlling the displacement of a pump and a valve having a valve member and two valve 60 chambers, one of which valve chambers is for connection, in use, to fluid at pump pressure and the other valve chamber is for connection to tank, the valve having two operating conditions in which, respectively, the servo-motor is influenced 65 by the pressure in the said one or the said other valve chamber and a neutral condition in which that influence is removed, wherein the arrangement is such that when, in use, the servomotor has placed the pump in its zero 70 displacement condition, the valve is in a condition in which the two valve chambers are connected.

2. A control device as claimed in claim 1, in which the valve comprises a single valve member.

3. A control device as claimed in claim 1 or 75 claim 2, in which the valve member is movable from a position, in use, corresponding to zero pump displacement in one direction against the force of a spring and in the opposite direction against the force of another spring, each spring gg being prestressed.

4. A control device as claimed in claim 3, in which the valve member is slidably movable in a bore provided between the two valve chambers and has an axial passage or recess at its periphery 85 which is longer than the bore.

5. A control device as claimed in claim 4, in which the bore is formed in a sleeve which is arranged with radial play in a housing part of the valve and is urged by one of the springs against a 90 wall separating the two valve chambers.

6. A control device as claimed in claim 5, in which the housing part is provided with an inner abutment, and the valve part is provided with an outer abutment, the diameter of the outer abutment being such that it can pass through the inner abutment, a respective annular disc being urged against each collar by a respective one of the springs, wherein when the abutments, in use, are in the same plane, the valve member is in a position corresponding to the pump being in a condition of zero displacement.

7. A control device as claimed in claim 6, in which the housing part is axially adjustable.

8. A control device as claimed in claim 7; in which the housing part has a screwthread enabling it to be axially adjustable.

9. A control device as claimed in any one of claims 6 to 8, in which each of the abutments is in the form of a collar.

10. A control device as claimed in any one of claims 6 to 9, in which each abutment is formed by a respective circlip.

11. A control device as claimed in any one of claims 1 to 10, in which the servo-motor comprises a piston/cylinder assembly and the valve member is slidably movable in a sleeve for defining the operating and neutral conditions, wherein the sleeve is attached to the piston.

12. A control device for adjustable fluid pumps or motors, comprising a servo-piston which adjusts a servo-piston which adjusts a servo-member of the pump, is arranged in a cylinder space and is subjected to a resetting force, and a valve which has a slide connected to an actuating element and a slide bore connected to the servo-piston and connects the pressure chamber of the servo-piston to a chamber at pump or motor pressure in its one operating position, to a chamber at discharge pressure in its other operating position and blocks in its neutral position, a central position of the slide corresponding to a position of the servo-member in which there is zero pump feed, characterised in that the slide forms a second valve in which, in the central position, connects the chamber at pump pressure to the chamber at discharge pressure.

13. A control device substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1980. Published by the Patent Office, 25 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.